1. Field of the Invention
This invention is concerned with an isomerization catalyst, a method for preparing the catalyst, and use of the catalyst in a process for selectively lowering the normal paraffin content of a hydrocarbon oil. In particular, it is concerned with a catalyst which is useful in an isomerization process for lowering the normal paraffin content of a hydrocarbon oil feedstock. The catalyst comprises an intermediate pore size silico-aluminophosphate molecular sieve and at least one Group VIII metal wherein said metal is occluded in the molecular sieve, i.e., the metal is added to the molecular sieve by coprecipitation of the sieve and metal components.
2. Description of the Prior Art
Shape-selective paraffin conversion of hydrocarbon oils is well known in the art and generally refers to the treatment of waxy hydrocarbon feeds to reduce the normal paraffins therein. The normal paraffin components of hydrocarbon oils, particularly long chain normal paraffins, impart, for many purposes, undesirable characteristics to the oils and hence must generally be removed, e.g., by catalytic dewaxing, in order to produce commercially useful products. In particular, middle-distillate and lube oil range hydrocarbon oils having high concentrations of normal paraffins, i.e., wax, generally have higher freeze points or pour points than oils having lower concentrations of normal paraffins. For many purposes it is desirable to have oils with low freeze points or pour points. Thus, for example, the lower the freeze point of a jet fuel, the more suitable it will be for operations under conditions of extreme cold. Thus, the fuel will remain liquid and flow freely without external heating even at very low temperatures. In the case of lubricating oils, it is desirable that the pour points be low, thereby enabling the oil to pour freely and adequately lubricate, even at low temperature.
The prior art shape-selective paraffin conversion processes, however, have the disadvantage of substantial cracking, and undesirably crack some of the potentially valuable hydrocarbon feedstocks to low value light products such as hydrocarbon gases.
Prior art shape-selective paraffin conversion catalysts dealing with paraffin cracking generally comprise an aluminosilicate zeolite having a pore size which admits the straight chain n-paraffins either alone or with only slightly branched chain paraffins, but which excludes more highly branched materials, cycloaliphatics and aromatics. Zeolites such as ZSM-5, ZSM-11, ZSM-12, ZSM-23, ZSM-35 and ZSM-38 have been proposed for this purpose in shape-selective paraffin conversion processes and their use is described in U.S. Pat. Nos. 3,700,585; 3,894,938; 4,176,050; 4,181,598; 4,222,855; 4,229,282; 4,247,388; 3,849,290; 3,950,241; 4,032,431; and 4,141,859.
Since shape-selective paraffin conversion processes of this kind function by means of cracking reactions, a number of useful products become degraded to lower molecular weight materials. For example, waxy paraffins may be cracked down to butane, propane, ethane and methane and so may the lighter n-paraffins which do not, in any event, contribute to the waxy nature of the oil. Because these lighter products are generally of lower value than the higher molecular weight materials, it would obviously be desirable to limit the degree of cracking which takes place during a catalytic shape-selective conversion process.
Prior patents dealing with paraffin isomerization include U.S. Pat. No. 3,432,568 which describes hydroisomerization of saturated aliphatic and cyclic hydrocarbons by contacting with a mixed dual-functional catalyst comprising hydrogen mordenite and a dehydrogenation component supported on a thermally stable carrier. U.S. Pat. No. 3,301,917 relates to hydroisomerization of paraffinic hydrocarbons in the presence of a mixed catalyst consisting essentially of an acid aluminosilicate portion and a hydrogenation component of a platinum metal supported on a thermally stable carrier. U.S. Pat. No. 3,673,267 describes a process for isomerization of paraffinic hydrocarbons under isomerizing conditions and in the presence of hydrogen with a catalyst of hydrogen mordenite having a silica to alumina mole ratio between about 20:1 and about 60:1, having associated therewith a metal of Group VIII, Group VIB or Group IB.
U.S. Pat. No. 4,419,220 discloses a process wherein hydrocarbon feedstocks are dewaxed by isomerizing the waxy component over a zeolite beta catalyst.
In isomerization processes, a principal problem is the attainment of high yield and selectivity of desired isomerate; and minimization of competing reactions is a consideration. A principal undesired competing reaction is cracking; and a common measure of effectivity of an isomerization catalyst is its ability to maximize isomerization while minimizing cracking.
An isomerization shape-selective paraffin conversion catalyst has now been found which effectively removes normal paraffins from a hydrocarbon oil feedstock by isomerizing them without substantial cracking. By use of certain silicoaluminophosphate molecular sieve catalysts which contain at least one Group VIII metal occluded therein, in the shape-selective conversion process, the normal paraffin content of hydrocarbon oil feedstocks may be effectively reduced wherein the products obtained are of higher molecular weight than those obtained using prior art aluminosilicate zeolites. The manner in which the Group VIII metal is associated with the silicoaluminophosphate is critical to obtaining the shape selective isomerization catalyst of this invention. The Group VIII metal is occluded within the pores of the silicoaluminophosphate by incorporating a water-soluble salt of the desired metal into the forming solution of the silicoaluminophosphate and then growing the silicoaluminophosphate crystals.
The catalyst of this invention is shape selective in that it isomerizes normal and slightly branched paraffins and does not essentially react with highly branched paraffins. Many of the prior art catalysts crack both the highly branched as well as the normal paraffins to lighter products and gases. Because these lighter products are generally of lower value than the higher molecular weight materials, it would obviously be desirable to limit the degree of cracking which takes place during the process.